FIG. 1 illustrates two cell clusters A and B forming part of a cellular mobile radiotelephone system in a manner well-known in the art. Typically, all of the frequencies of a system are used in each cell cluster. Within the cell cluster, the frequencies are allocated to different cells so as to achieve the greatest uniform distance, known as the frequency reuse distance, between cells in different clusters using the same frequency. In FIG. 1, cells A.sub.1 and B.sub.1 both use a common frequency as do cells A.sub.2 and B.sub.2, cells A.sub.3 and B.sub.3, etc. The radio channels in cells A.sub.1 and B.sub.1 using the same frequency are referred to as co-channels because they share the same frequency. Although some interference will generally occur between co-channels, the level of such interference in an arrangement such as that of FIG. 1 is normally acceptable. The cell plan of FIG. 1 therefore allows for a relatively simple frequency allocation and provides for an acceptably low level of co-channel interference.
It is also well-known in the art for radio base stations located near the center of each cell (or near the center of three adjacent "sector cells") to provide radio coverage throughout the area of the cell as illustrated in FIG. 2. The base station consists in part of a number of channel units, typically including a single control channel CC, a number of voice channels VC, and a single strength receiver SSR. For convenience, only three cells are illustrated in FIG. 2.
The cell plan of FIGS. 1 and 2 assumes a relatively uniform distribution of mobile radiotelephone users throughout the area of a cell. To handle relatively dense concentrations of mobile users, a preferable arrangement is to establish localized "microcells" as illustrated in FIG. 3. Microcells allow additional voice channels to be physically located in close proximity to where they are actually needed, boosting cell capacity while maintaining low levels of interference. Microcells may cover thoroughfares such as crossroads or streets, and a series of microcells may provide coverage of major traffic arteries such as highways. Microcells may also cover large buildings such as a football stadium or a shopping mall, for example.
A mobile station chooses a cell to access by selecting the cell having the strongest control channel as received at the mobile station. A mobile station in FIG. 3, for example, may choose to access a microcell or may choose to access an umbrella cell, assuming all base stations are provided with transmitting control channels, an umbrella cell being defined as a cell covering one or more other cells.
Although microcells provide an attractive way to increase cell capacity, they also pose a number of problems from the standpoint of cell planning and frequency allocation. One problem that arises with the increasing use of microcells is that the number of control channel frequencies available in the system is limited. Out of typically 300 to 400 available frequencies, 21 are normally allocated for use as control channels. The 21 control channel frequencies are normally extensively reused in a 21 cell repeating pattern similar to the smaller 7 cell repeating pattern of FIG. 1. In introducing microcells in cells that are already part of a 21 cell repeating pattern, great care must be taken to select a control channel for use by the microcell that will not result in intolerable interference. Regardless, interference between control channels inevitably increases. If more than the normal 21 control channel frequencies are used for control channels, less channels will be available for voice/speech channels. If each microcell uses a control channel, the relative amount of hardware for control channels is higher than in conventional cells, since the microcell is likely to be equipped with fewer voice channels than the conventional cells. Another problem arises because of the existing structure of the air interface between the base station and the mobile stations. If a cell is congested, i.e., all of the cell's voice channels are occupied, additional would-be users are directed to try other nearby cells. According to the present structure of the air interface, a maximum number of 6 cells may be suggested to be investigated by the user. Absent microcells, the 6 suggested cells would be the 6 neighboring cells of a given cell in the regular hexagonal pattern. With the increasing use of microcells, more than 6 microcells might be located within a single umbrella cell. Determining the composition of the cell list sent in connection with a directed retry instruction therefore becomes difficult. If a microcell may satisfactorily handle a call, interference is generally reduced. The coverage of microcells, however, is very limited. Furthermore, the microcells may have a relatively small number of voice channels compared to the umbrella cell so that congestion at a particular microcell frequently occurs. Under such circumstances, it is difficult to determine when reference to a microcell should be substituted for a reference to a neighboring umbrella cell in a directed retry message. When the capacity of the microcells is small, the frequency of directed retry messages increases.
Another problem involves the street corner effect that occurs when a mobile station travelling in a direction of coverage of a microcell changes directions so as to abruptly pass beyond the coverage of the microcell. In order to remain in contact with the system to receive possible calls, the mobile station must scan the designated control channel frequencies to determine a strongest one. During such rescanning, the mobile station is effectively "deaf", unable to respond to the system for some 10-15 seconds in a typical case. As the patchwork of microcells becomes more complex, the mobile station in the worst case may find itself continually rescanning the control channels and continually out of touch with the system.
One possible solution to the foregoing problems is to not provide a microcell with any control channel whatsoever. In such an instance, call set-up would be exclusively handled by the umbrella cell and traffic could be handed off afterward, preferably as fast as possible, to the appropriate microcell (if there is a microcell with a free voice channel) following a locating procedure. If the number of microcells is large, however, a lot of traffic would have to transit through the umbrella cell. A risk is posed that congestion will occur in the umbrella cell despite there being free capacity in one or more microcells. In addition, many calls would experience one extra hand-off, causing increased noise and risking the call being dropped.
What is needed is an arrangement that allows for the extensive use of microcells to increase system capacity but that does not complicate cell planning or limit the use of directed retry and does not result in excessive rescanning by the mobile stations.